1. Field of the Invention
The present invention relates to a power supply system adapted for use in an imaging equipment such as a video camera system, with an interchangeable lens.
2. Related Background Art
Imaging equipment including video camera systems has shown remarkable progress in recent years, and, for improving the performance particularly in video cameras and camcorders, there have been proposed interchangeable lens systems capable of using various lenses.
In the field of interchangeable lens systems, those for single lens reflex cameras (lens systems for still cameras) are already well known, and various lenses including special ones for specific purposes are already made commercially available.
However the video camera systems are still in the introductory period, and the market therefor is still undeveloped, so that the manufacture of many special lenses featuring the interchangeable lens system is unpractical in such initial stage. On the other hand, it will be very convenient for the users if the commercially available interchangeable lenses for the single lens reflex cameras can be used on the video cameras. Thus, with the recent spreading of video cameras, there is an increasing desire for using the interchangeable lenses for single-lens reflex cameras also for the video cameras.
For this purpose, there is required a conversion adapter for connecting the lenses for still camera with the video camera.
Such conversion adapter is required:
(1) for matching between the lens mount in the interchangeable lens system of the video camera with that of the lens system of the still camera;
(2) for compensating the difference in the optical path length from the lens mount to the image plane, between the video camera and the still camera; and
(3) for matching the communication format, data format and/or control protocol in the control data transfer, between the interchangeable lens system for the video camera and that for the still camera.
In general, in the interchangeable lens system for the video camera, the electric power to the lens is supplied from a battery or an AC adapter connected to the video camera, through electrical contacts provided on the lens mount.
Since the video camera generally includes a driving circuit which requires a large electric power in continuous manner, such as a motor, the electric power supplied to the lens is often subjected to a certain limitation, in consideration of the usable time of the battery.
On the other hand, in the still camera system, the limitation on the electric power is relatively not strict, because the driving circuit requiring a large electric power, such as a motor, is not used in continuous manner.
As a result, in case the lens for still camera is connected to the video camera, the electric power required by said lens may exceed the maximum electric power supply of the video camera.
For filling such deficiency in electric power supply, it is conceivable to provide the conversion adapter with another electric power supply source such as an external battery.
Such configuration functions normally when the conversion adapter is connected to the video camera, but results, firstly, in a drawback that the electric power from said supply source is supplied to and wasted in the lens, in case said conversion adapter is not connected to the video camera but only to the lens. The battery will become completely discharged and unusable if the system is left in this state.
Secondly, such external battery has to be compact and light, because it is connected to the conversion adapter and used in combination with the video camera, the conversion adapter and the lens for still camera when the system is actually used by the user. For this reason, there is required a compact primary battery as ordinarily employed in the still camera systems, such as a lithium battery. In the use of such primary battery, the power consumption has to be controlled more strictly than in ordinary systems, since the battery is not re-usable.